1. Field of the Invention
The present invention relates to a deflection apparatus for use with a color cathode-ray tube, and more particularly to a deflection apparatus which is able to correct a convergence satisfactorily by improving an interference between a coma aberration correction coil and a convergence correction coil.
2. Description of the Related Art
FIG. 1 of the accompanying drawings is a side view showing a structure of a cathode-ray tube having a deflection apparatus according to the related art. In a display apparatus using a cathode-ray tube, a deflection yoke 22 is disposed in a funnel portion 21 B of a cathode-ray tube 21 as shown in FIG. 1.
The deflection coil 22 comprises an annular core 23, a horizontal deflection coil (not shown), a vertical deflection coil (not shown), or the like.
An inline type electron gun 24 is attached to a neck portion 21A of the cathode-ray tube 21. The inline type electron gun 24 emits three electron beams for displaying red (R), green (G) and blue (B) colors.
The circuit provided in the display apparatus may cause a sawtooth current of a horizontal deflection period to flow through a horizontal deflection coil and may cause a sawtooth current of a vertical deflection period to flow through a vertical deflection coil.
The electron beams may be deflected in the horizontal direction by the horizontal deflection coil and may be deflected in the vertical direction by the vertical deflection coil. Thus, the electron beams may scan a phosphor screen 27 and an image is displayed by resultant light.
Generally, the vertical deflection coil is designed so as to generate a barrel-shaped vertical deflection magnetic field. The barrel-shaped magnetic field has a nature such that its magnetic field becomes strong in the starting point or the ending point of a magnetic line of force. The cathode-ray tube having the inline-type electron gun may be generally designed such that a center electron beam may be used to display green color and left and right electron beams may be used to display blue and red colors.
As shown in FIG. 2, in the barrel-shaped vertical deflection magnetic field, blue display and red display electron beams may be strongly affected in the vertical direction as compared with the green display electron beam. In other words, the center electron beam may have a vertical deflection amount smaller than those of the left and right electron beams. As a consequence, the upper and lower areas of the green raster may become slightly smaller than those of the red and blue rasters. This phenomenon can be understood such that the deflection amounts of three electron beams may become different due to a coma aberration.
In order to solve the problem of the above-mentioned phenomenon, there may be used a coil which might be called a coma aberration correction coil.
As shown in FIG. 1, a coma aberration correction coil 25 may be attached to the rear portion of the deflection yoke 22.
Alternatively, as shown in FIG. 4, the coma aberration correction coil 25 comprises C-shaped cores 26 and 26' symmetrically disposed in the upper and lower direction and coils 27 and 27' wound around the C-shaped cores 26 and 26'.
As shown in FIG. 5, the coma aberration correction coil 25 may be connected to the vertical deflection coil in series. Accordingly, a vertical deflection current flows through the coma aberration coil 25. A magnetic field generated from the coma aberration correction coil 25 may become a dipole magnetic field shown in FIG. 6. This dipole magnetic field may achieve effects which can cancel a coma aberration caused by the influence of the vertical deflection magnetic field.
Further, in FIG. 5, by changing a balance of amounts of currents flowing through the coils 27 and 27', it is possible to correct a misconvergence of a pattern shown in FIG. 7. To this end, there may be frequently used a means, such as a resistor 28 shown in FIG. 5, for changing a balance of amounts of currents flowing through the coils 27 and 27'. However, the above-mentioned method may be unable to correct a misconvergence of a pattern different from that shown in FIG. 7. Therefore, a misconvergence may be corrected by using another coil different from the coma aberration correction coil.
In this case, another coil, i.e., a convergence correction coil, also is disposed at the rear portion of the annular core 23, i.e. the electron gun 24 side. Misconvergences of various patterns can be corrected by causing a parabola current of a deflection period, or the like, to flow through the convergence correction coil.
However, in order to simply the structure of the deflection apparatus, the convergence correction coil may be attached so as to use the cores 27, 27' in common. In this case, a magnetic coupling between the coma aberration correction coil 25 and the convergence correction coil may be strengthened. Thus, when the parabola current or the like flows through the convergence correction coil, the induced current occurs in the coma aberration correction coil. Consequently, there arises such a problem that a correction effect achieved by the convergence correction coil will be decreased unavoidably.